Measuring RPM with LabVIEW

There appears to be two ways of measuring the rotational speed with Labview, one of which is easier than the other. First things first let me explain how the rotational speed of a micro-gas turbine can be found as there are some important subtleties between each technique.

The 'old school' method way back when I first started building jet engines was to use the case pressure of the engine and a compressor characteristic performance map. These performance maps are readily available from turbocharger manufacturers and since most micro-gas turbines are built around a turbocharger compressor this would seem to give a good approximation. But, as with all things to do with jet engines, the simplicity of this idea isn't as simple when you need to use it. Looking at the Wren MW54 which uses a turbocharger compressor from Garrett, (Part number 446335-9), which is also is used on the GT2854R turbocharger. Although the compressor may be identical the second stage of the compressor is different. The MW54 uses a series of wedges for the stator blades of the compressor stage, while the turbocharger uses a bladeless spiral volute. The reason for this is to provide greater efficiency over a wide range of rotational speeds required for a pistion engine, but the increased size, area and weight means that this is a no-no for small model aircraft gas turbines. This means that the compressor map wont truly represent the behaviour of the compressor, and in my experience with the vast amount of experimentation and modelling of these small engines I can say that the average turbocharger compressor map is at best within + 6% of the performance of the compressor used in a gas turbine engine. Now 6% does not sound like much, but if your engine has a mechanical limit of 100K RPM then I will let you figure out how big that error is, and the value of 6% is based entirely on the MW54 engine. Other engines showed as much as 10% deviation from the predicted performance shown on the compressor map. Of course if you have modified a turbocharger into a gas turbine then you wont have this problem.

A typical compressor map looks like the chart shown on the right. It is actually a plot of four non-dimensional values and if you're handy with non dimensional analysis then list all the variables for all the fluid properties that a compressor will experience and you will derive four equations. These are known as corrected massflow, corrected rotational speed, a pressure ratio across the compressor and the isentropic efficiency of the compressor. All the work I have previously seen with amateur gas turbines and in the model gas turbine book by Thomas Kamps, (good general book for beginners but I think a lot of the mathematics was lost in translation from German to English), simply states that you can use the values as given, but the corrected speed and massflow need to be modified with the equations below, before you can use them.

So why am I telling you this, well simply knowing the pressure is not enough to find the corresponding rotational speed or massflow. You will need at least three known values before you can locate the operating point on the compressor map. This is a complex problem and requires complicated computational models to solve them, something that has put years of wrinkles on my face from lack of sleep. But you can take several pressure and temperature measurements at various throttle settings and take a best guess at a 'steady state operating line' shown in dashed red in the picture above. This will give you a line which you get approximate RPM, massflow and effiency values from a known pressure ratio.

The second method was to use some some of frequency counting to measure the rotational speed of the engine. This was commonly done by using infra-red LED's and has now moved onto Hall sensors. The infra-red method works by passing an infra-red beam through a hole in the compressor nut, while the Hall sensor works by detecting the pulse generated from a passing magnet embedded within the compressor nut. Both methods generate a signal that can be approximated to a square wave signal, and the frequency is the rotational speed of the engine.

Now Labview has the necessary functions in which to find the frequency of a signal and in fact I came up with at least four methods which worked with any number of simulated signals, with noise over any frequency from around 10Hz to 2500Hz. Seems ok, but that equates to RPM values of 600 to 150000, not good since I want to measure the RPM from zero up to at least 165000. When I came to test the tachometer with both a signal generator and an actual RPM signal from the engine it didn't work, no matter what much I tried. Labview has what they call an oscilloscope function that you can use to analyse the behaviour of a signal while the program is running and this showed I had a valid signal, and in fact using a premade Tachometer function block which opens up into a nice GUI showing a speed over time graph and a waveform of the signal but it would not output anything!

Hmm, I think I  need much, much, much more experience with Labview but time is short and I have already decided to go another route. Rather than measuring the signals frequency I will convert the frequency into a linear analogue voltage signal using a National Instruments LM2917 chip. The data sheet accompanying this chip and a trawl through Google showed that most people use this chip to build car tachometers measuring low RPM's with multiple pulses per rotation. After a coffee and sitting down to read the data sheet properly resulted in the page of maths in the picture on the right and an order for several bags of capacitors and other bits and pieces. The circuit is based on a maximum frequency of 3500Hz which is 210000 RPM, which should help to keep the results in the linear region of the chips output. The output is also designed to be between 0 and 5V, which is line with all my other analogue voltage signals. This may work, it may not, time will tell once I get my new oscilloscope I will be able to investigate this method better and see if it actually works. Stay tuned!

Edit: The NI LM2917 ic can be found on EBay for £1.40 so cheap enough to experiment with.